1. Field
Embodiments of the invention relate generally to a nonvolatile semiconductor memory device and a method for manufacturing the same.
2. Background Art
Nonvolatile semiconductor memory devices (memory) conventionally have elements integrated in a two-dimensional plane on a silicon substrate. Although the dimensions of one element are reduced (downscaled) to increase the memory capacity of the memory, such downscaling in recent years has become difficult in regard to both cost and technology.
On the other hand, although many ideas for three-dimensional memory, etc., have been proposed, three-dimensional devices generally require processes including at least three lithography processes for each layer. Therefore, costs cannot be reduced even when three dimensional; and instead, stacking four or more layers undesirably leads to cost increases.
Conversely, collectively patterned three-dimensionally stacked memory cells have been proposed (for example, refer to JP-A 2007-266143 (Kokai)). According to such a method, it is possible to collectively form a stacked memory without increasing the number of stacks. Therefore, it is possible to suppress cost increases.
In such a collectively patterned three dimensionally stacked memory, a stacked unit is formed on a silicon substrate by alternately stacking insulating films with electrode films forming word lines, and then collectively making through-holes in the stacked unit. Then, for example, a charge storage layer (a memory layer) is provided on the side faces of the through-holes; and silicon pillars are provided on the inner sides thereof. A tunneling insulating film is provided between the charge storage layer and the silicon pillars; and a blocking insulating film is provided between the charge storage layer and the electrode films. Thereby, a memory cell is formed at the intersection between the silicon pillars and each of the electrode films.
Further, two of the through-holes may be connected by a connection portion provided on the silicon substrate side to form a silicon pillar having a U-shaped configuration. In other words, a memory string made of a silicon pillar having a U-shaped structure can be formed by making a through-hole having a U-shaped configuration, forming a blocking insulating film, a charge storage layer, and a tunneling insulating film on the side wall thereof, and filling silicon into the remaining space.